Walking a pet on a retractable leash can be problematic with pets that pull or lunge. Similarly, a pet tied to a run may leap against the restraint. Because the restraint is inelastic, the pet can be subject to a jerking force on the pet's neck. This jerking can result in neck injuries to the pet if on a collar, and other injuries even if on a harness, such as muscle tears. A large dog lunging can put repeated strain or shock on the pet owner's arm, wrist or shoulder.
What is needed is a device that progressively encourages the pet not to strain against the leash or restraint, in a safe and less injury prone manner than just a leash. By progressive, what is meant is that the harder the pet may pull, the more the pet should be encouraged not to pull.